The invention relates to an electro-optical light-modulation element consisting of an electro-optical crystal having flat sides which are parallel to one another. Transparent first electrodes are situated on transparent intermediate layers on each side of the element.
In the above-described electro-optical light modulation element the light is phase modulated by using the longitudinal electro-optical effect (Pockels effect). This effect occurs in a monocrystalline plate of a material whose optical axis extends perpendicular to the surface of the plate and which belongs to the potassium hydrogen phosphates family. When such a crystal is placed in an electric field E directed parallel to its optical axis (crystal axis) c, a difference .DELTA.n, between the refractive indices for components, of a light ray travelling in the direction of the optical axis, which are polarized at right angles to each other, is produced. These light components which initially are of equal phase show a phase difference .phi. when they leave a crystal of thickness d. Since the effect is linear, .DELTA.n is proportional to the field E and .phi. is proportional to the product E. d. Thus, .phi. is proportional to the potential difference V between the input and output faces of the crystal. Accordingly, the polarization of a linearly polarized light beam can be influenced by means of such a crystal. (Optica Acta, Vol. 24, No. 4, pp. 413- 425, 1977.)
Light modulation elements of this type can be used, for example, in various systems such as polarization analysis apparatus, electrically controlled light shutters, and apparatus for measuring the thickness and the index of refraction of films.
Such a light modulation element is already known from German Offenlegungsschrift No. 24 29 813. Intermediate layers for insulating the crystal against ambient moisture are present between the flat parallel sides of the electro-optical crystal and the electrodes. The electrodes themselves consist of semitransparent metal films of high electric conductivity, for example gold, silver, and copper, which are placed directly on the intermediate layers. In the case of gold the electrodes are approximately between 5 nm and 16 nm thick. Both the intermediate layers and the electrodes are provided on the crystal by vapor deposition methods.
Although the electrodes have a very high electric conductivity, their transparency is small. Therefore, such light modulation elements are not especially suitable for phase modulation of light beams of low intensity.
For the manufacture of highly conductive and simultaneously highly transparent electrodes it is already known (see German Offenlegungsschrift No. 24 29 813, p. 3, paragraph 2) to provide a transparent conductive film of titanium oxide or indium oxide uniformly on the light surfaces of the electro-optical crystal. For example, in order to obtain such electrodes on the surfaces by vapor deposition, the crystal itself should be heated at a temperature of at least 150.degree. C. for a rather long period of time. However, when an optical crystal, for example, a KDP crystal, is heated (tempered) in such manner, then its electro-optical properties are destroyed.
It has also been suggested to provide highly conductive and highly transparent electrodes, for example of indium oxide, on a substrate of, for example, glass. The high temperatures required for this purpose would not damage the substrate (for example, a flat glass plate). However, in order to contact the electrodes with the crystal, the glass plate should be cemented to the crystal. However, this would not produce a sufficiently homogeneous electric field inside the crystal. Nonuniform field distributions might result at the interfaces between the crystal and the glass as a result of barrier layer formations. Furthermore, nonuniform field distributions might arise by nonuniform conductivity of the crystal or by wedge-shaped cement layers and bubbles in the layers of cement.